4
;Pause 150 mSec, before making measurements, so the
;probe can stabilize on true readings.
;
03: Excitation with Delay (P22)
1: 1 Ex
Channel
2:
0
Delay W/Ex (units = 0.01 sec)
3:
15
Delay After Ex (units = 0.01 sec)
4: 0 mV
Excitation
;Measure the HMP45C212 relative humidity.
;
04: Volt (SE) (P1)
1: 1 Reps
2:
5
2500 mV Slow Range
; CR500 (2500 mV); CR23X (1000 mV); 21X, CR7 (5000 mV)
3:
4
SE Channel
; Green wire (SE 4)
4:
2
Loc [ RH_pct ]
5:
.1
Mult
; mult 0.1 for percent RH, mult 0.001 for fraction
6: 0 Offset
;Turn the HMP45C212 off.
;
05: Do
(P86)
1:
51
Set Port 1 Low
; Yellow wire (C1)
7. MAINTENANCE
The HMP45C212 Probe requires minimal maintenance.
Check monthly to make sure the radiation shield is free
from debris. The black screen on the sensor’s end
should also be checked.
When installed in close proximity to the ocean or other
bodies of salt water (e.g., Great Salt Lake), a coating of
salt (mostly NaCl) may build up on the radiation shield,
sensor, filter and even the chip. NaCl has an affinity for
water. The humidity over a saturated NaCl solution is
75%. A buildup of salt on the filter or chip will delay or
destroy the response to atmospheric humidity.
The filter can be rinsed gently in distilled water. If
necessary, the chip can be removed and rinsed as well.
Do not scratch the chip while cleaning.
Long term exposure of the HUMICAP® relative humidity
sensor to certain chemicals and gases may affect the
characteristics of the sensor and shorten its life. Table 2
lists the maximum ambient concentrations, of some
chemicals, that the HUMICAP® can be exposed to.
Detailed information on allowed concentrations can be
requested from Campbell Scientific representatives.
Table 2. Chemical Tolerances of HMP45C212
Chemical
Concentration (PPM)
Organic solvents
1,000 to 10,000
Aggressive Chemicals
(e.g. SO
2
, H
2
SO
4
,
H
2
S, HCL, CL
2
, etc.)
1 to 10
Weak Acids
100 to 1,000
Bases
10,000 to 100,000
Recalibrate the HMP45C212 annually. Obtain an
RMA number before returning the HMP45C212 to
Campbell Scientific for recalibration.
8. REFERENCES
Goff, J.A. and S. Gratch, 1946: Low-pressure
properties of water from -160° to 212°F,
Trans.
Amer. Soc. Heat. Vent. Eng.
,
51
, 126-164.
Lowe, P.R., 1977: An approximating polynomial
for the computation of saturation vapour
pressure,
J. Appl. Meteor.
,
16
, 100-103.
Weiss, A., 1977: Algorithms for the calculation of
moist air properties on a hand calculator,
Amer.
Soc. Ag. Eng.
,
20
, 1133-1136.
